Mitochondrial dysfunction has been implicated in age-related neurodegenerative diseases and may play a role in the decline of cognitive and sensory function with healthy aging. However there is no direct in vivo evidence for altered brain mitochondrial function. Over the last decade we have developed non invasive 13C Magnetic Resonance Spectroscopy (MRS) methods, in conjunction with stable 13C isotope labeled substrates, to study brain metabolism in humans. MRS has the unique ability to non-invasively measure the rates of the neuronal and glial TCA cycles - a direct measure of in vivo mitochondrial oxidative energy production. Using 13C MRS we found profound alterations in energy metabolism in the occipital lobe of healthy elderly subjects including a 28% reduction in the neuronal TCA cycle, a parallel 24% reduction in the glutamate/glutamine cycle, and a 30% increase in the glial TCA cycle. Our general hypothesis is that in healthy aging there is a loss of capacity of neuronal mitochondria to support brain functional energetic requirements. We will address three questions in the proposed research - 1) Are these metabolic changes present in the prefrontal cortex, which has been implicated in the loss of cognitive function with aging? 2) Does the severity of impairment of the neuronal TCA cycle correlate with performance on an established test of executive function and 3) are the alterations in glial mitochondrial metabolism due to enhanced glutamate oxidation and anaplerosis, which may reflect impaired synaptic glutamate clearance. Answering these questions will have immediate significance in understanding the role of mitochondrial function in the cognitive declines associated with normal aging, potentially provide novel, non-invasive biomarkers of this process, and also will provide critical baseline information for the application of these methods to study the role of mitochondrial dysfunction in the development of Alzheimer's and other neurodegenerative disorders.